Extract key binding and selection logic into a KeyBindingResponder struct

[davidbalbert]

Only do this after we support multiple selections #106.

The ideal is that every method in TextView+KeyBinding.swift and TextView+Mouse.swift should be a single line.

Design thoughts:

• KeyBindingResponder should have an associated method for each method on NSStandardKeyBindingResponding
• It should handle things like scrolling the selection into view, autoscroll, etc.
• The owning view should provide or be a KeyBindingResponderDelegate to do the actual scrolling, receive edits, etc.
• For transpose(_:) and transposeWords(_:) it should be possible to control whether the transposed characters get selected or not.
• Don't rely on any globals. Even an NSApplication instance should be passed in if necessary.